Rubber compounds which constitute the starting material of each elastomeric product such as tires, conveyor belts, seals, elastic bellows and the like are complex reactive multicomponent systems which generally comprise rubber, processing (softener) oil to stretch the polymer and to improve the processing characteristics, chemicals for obtaining special characteristics such as fire protection and light protection, bonding agents, reinforcing filler substances such as soot, silicic acid or fibers for increasing the mechanical strength, and vulcanizing systems comprising vulcanizing agents, activators, accelerators and retarders.
Because of the different forms (lumps, granular material, powder, liquid), the extremely different viscosities and the very considerably different parts by weight (for example, one part sulfur to 100 parts rubber), the mixing of these individual substance components constitutes a method and machine task which is most difficult. In addition, a special significance is imparted to the process for producing the compounds since the physical and chemical characteristics of an elastomeric manufactured product is not only dependent upon the formulation of the rubber compound but also on the compounding technology. Accordingly, the sequence of metering of rubber, soot and softener has a considerable influence on the soot dispersion and therefore on the mechanical strength of the manufactured part.
According to the present state of the art, rubber compounds are almost exclusively produced in closed mixers characterized as masticators.
A closed mixer assembly comprises a metering system, weighing system and transport system as well as the closed mixer, an extruder or roller assembly for converting the charge of the closed mixer to rubber sheets, a refrigerating unit for the sheets and a positioning or cutting device.
The actual compounding process takes place in a chamber having two closable openings and two rotors rotating in mutually opposite directions which are arranged parallel and equipped with masticating elements. The walls of the compounding chamber, the rotors, the charging ram and the discharge saddle flap are temperature controlled by circulating liquid. The substance components are supplied with an actuated ram via the feed shaft while liquids are injectable via nozzles directly into the mixing chamber. The edges of the rotating masticating blades define gaps in which the material is sheared and dispersed. The masticating elements are so configured that the mixing charge is forcibly conveyed in the longitudinal and peripheral directions. The flow division resulting from the laminar flow field and the rearrangement cause a distributive mixing effect. At the end of a cycle, the mixing chamber is opened by pivoting the saddle flap and the compounded piece is discharged by the conveying action of the rotors.
The holding time or cycle time required to obtain a homogeneous compound is empirically determined for each formulation. The process is controlled to end when the pregiven values of the time, rotor torque, rotor revolutions, the temperature of the material being mixed or the energy supplied are reached.
The energy supplied via the rotors is for the most part dissipated in the highly viscous polymer mass. Because of the thermally unfavorable surface/volume ratio of the mixing chamber, the dissipation heat can only be transferred to a limited extent. The temperature increase of the material being mixed resulting therefrom forms the basis of one of the most serious disadvantages of the closed mixing process, namely: in order to avoid a premature start of the temperature-dependent vulcanizing reaction, a rubber compound must generally be produced with a closed mixer in several steps. In the first step, all non-reactive constituents are mixed. In the case of high concentrations of a charging substance and thermal instability of the polymers, several cycles can already be required for this purpose.
The premix characterized as the base compound is cooled down from approximately 100.degree. C. to 160.degree. C. to 20.degree. C. to 40.degree. C. after being discharged from the closed mixer and thereafter supplied to the end compound stage. There, the reactive vulcanizing chemicals are mixed into the base compound likewise with a closed mixer with a temperature of approximately 80.degree. C. to 120.degree. C. not being permitted to be exceeded. The end compound contains all constituents after this stage and is, in turn, cooled down to 20.degree. C. to 40.degree. C. In many cases, the end compound is stored in advance of further processing for 20 to 40 hours in order to raise the quality of the material being mixed to the required level via diffusive material transport during this storage time.
The production of rubber compounds with closed mixers is burdened with two substantial disadvantages. On the one hand, a closed mixer can principally be driven only discontinuously. From this, an interruption in the continuous manufacturing sequence results with organizational and logistical problems. On the other hand, the danger is present of fluctuations in quality because start-up effects occur with each cycle because of the intermittent operation.
Secondly, the temperature of the material being mixed is influenced only within narrow limits and it is for this reason that the mixing process must be carried out in several stages and, after each mixing stage, a reshaping and cooling must take place. Because of the multiple stages, the following consequences occur: the mixing time is lengthened, a very large energy requirement has to be satisfied because of the repeated plasticization and cooling of the compound and the necessity arises of providing storage and transport capacities for the intermediate compounds.
Because of these disadvantages of the closed mixer, the development of alternate mixing apparatus was begun already in the 1960s. Thus, the transfer mix extruder known, for example, from published German patent application DE-AS 11 42 839 was investigated as to its suitability as a rubber mixer. The foregoing concerns a single-screw extruder wherein not only the rotating screw but also the stationary cylinder has conveyance paths. The material is transferred several times from the conveying paths of the screw into those of the cylinder and back whereby a good mixing effect is obtained. However, because of thermal reasons, the combination of the production of rubber base compounds and rubber end compounds in a continuous process sequence and achieving this in only one mixing arrangement was not possible.
A twin-shaft machine known from published German patent application DE-AS 16 79 829 is likewise suited only for end mixing. A Farrel continuous mixer comprises essentially two shafts journalled at opposite ends and rotating in mutually opposite directions with the shafts being configured as conveying screws in the input area and as masticating blades in the output area. The holding time can be influenced via the rotational speed of the shafts and a throttle in the output cross section.
Furthermore, a single-shaft machine developed by Buss AG and characterized as a co-masticator has become known for producing rubber compounds. In this single-shaft machine, the blades of the screws in the co-masticator are penetrated over the surface in a manner comparable to a pin extruder. During the rotation, these breakthroughs engage masticating teeth fixed in the cylinder. To increase the mixing action in the longitudinal direction, the shaft carries out an oscillating longitudinal movement with each rotation (Kautschuk und Gummi, Kunststoffe, 38th year, no. 2/85, pages 116 to 121).
Finally, a method and an arrangement for producing rubber compounds is known from U.S. Pat. No. 5,011,291 wherein two closed mixers are arranged in series. The first closed mixer supplies the base compound of the rubber while the second closed mixer takes over the task of the end compound. The second closed mixer is located directly below the mixer for the rubber base compound so that the rubber base compound can be guided directly into the finish mixer as a charge and can be further processed there.
It is true that with this method procedure the very costly cooling and reheating via the intermediate storage of the base compound is avoided, it is however disadvantageous, as with all other methods and arrangements according to the state of the art, that a continuous production of rubber end compounds is not possible. In addition, the capability of cooling the second closed mixer operating as a finish mixer is poor as before because of its comparatively poor ratio of coolable surface to the quantity being mixed disposed in the mixer.
As an alternative to the closed mixer, in actual practice no alternative could prevail for overwhelming technical reasons such as inadequate mixing capacity or thermal problems. The state of the art of producing rubber compounds is as before the closed mixer developed on the basis of a dough masticating machine.